Cellular mechanisms governing the regulation of pulmonary vascular tone are complex and incompletely understood, particular during pathophysiological conditions. One such condition is hepatopulrnonary syndrome. Hepatopulmonary syndrome is a clinical triad of advanced liver disease (usually cirrhosis), pulmonary gas exchange abnormalities (i.e. shunting) leading to severe systemic arterial hypoxemia, and widespread pulmonary vascular dilatations in the absence of intrinsic cardiopulmonary disease. This syndrome occurs in 15% of cirrhotic individuals and vastly complicates their treatment. Nitric oxide (NO) has been postulated to be central to the development of hepatopulmonary syndrome. Ligation of the common bile duct in rats is a recently developed animal model that has proven useful for investigating the pathogenesis of hepatopulmonary syndrome. These animals have intrapulmonary shunting and are hypoxemic. The mechanisms linking NO to the development of hepatopulmonary have not been defined. This proposal investigates the underlying mechanisms of hepatopulmonary syndrome using a comprehensive approach of in vivo and in vitro experimental strategies. We provide preliminary data demonstrating that in addition to elevated NO and eNOS, expression in lung of the vasoconstrictor endothelin (ET-1) is decreased in cirrhotic rats. Also, the stress response gene, heme oxygenase (HO-1), is significantly upregulated in lungs of cirrhotic rats. HO-1 enzymatic activity liberates carbon monoxide (CO), a known vasodilator that can act via cGMP-dependent and -independent pathways. Our functional physiological evidence suggests that the chronic elevation of NO leads to alterations to the pulmonary vasculature beyond NO's ability to act as a vasodilator. We will test the hypotheses that: 1) Chronic NO elevation during cirrhosis renders the pulmonary circulation unresponsive to hypoxia and that this can be reversed by inhibiting NO production or in NOS-knockout mice; 2) Chronic NO elevation activates pulmonary artery vascular smooth muscle cell K+ and/or Cl- channels hyperpolarizing these cells and reducing their contractile ability; and 3) HO-1 induction is caused by NO further contributing to pulmonary vasodilation. This project will not only define the cellular basis for hepatopulmonary syndrome, but will also contribute to our understanding of how pulmonary vascular tone is controlled at the most basic level.